Chemical sensing science is a field with many applications in industry, security, and defense. A multitude of detection mechanisms exist, but many are only applicable to very specific situations. Generally, effective chemical sensing needs to be highly selective to a target analyte and result in a large electrical response.
Transducing a chemical response into an electrical response can be done most directly with a chemiresistive polymer, in which the reaction either acts to dope or undope a semiconducting polymer [J. Janata and M. Josowicz, “Conducting polymers in electronic chemical sensors,” Nature Materials, vol. 2, pp. 19-24, January 2003] or cause a conductivity change by swelling a partially conductive polymer [D. Rivera, M. K. Alam, C. E. Davis, and C. K. Ho, “Characterization of the ability of polymeric chemiresistor arrays to quantitate trichloroethylene using partial least squares (PLS): effects of experimental design, humidity, and temperature,” Sensors and Actuators B-Chemical, vol. 92, pp. 110-120, July 2003].
However, detection of an analyte may also be done mechanically, such as with a microcantilever. Microcantilevers are a common platform for transducing a chemical response into a mechanical one. One surface of the cantilever is coated with a material (often a polymer) that the target chemical will interact with, typically through absorption (swelling) or reaction. The interaction creates a stress that causes the cantilever to bend. Existing methods rely on measuring small cantilever deflections of less than a few microns [N. V. Lavrik, M. J. Sepaniak, and P. G. Datskos, “Cantilever transducers as a platform for chemical and biological sensors,” Review of Scientific Instruments, vol. 75, pp. 2229-2253, July 2004] or measuring resonance frequency changes due to a mass increase. In order to measure these small changes, optical [F. M. Battiston, J. P. Ramseyer, H. P. Lang, M. K. Baller, C. Gerber, J. K. Gimzewski, E. Meyer, and H. J. Guntherodt, “A chemical sensor based on a microfabricated cantilever array with simultaneous resonance-frequency and bending readout,” Sensors and Actuators B-Chemical, vol. 77, pp. 122-131, June 2001], piezoelectric [J. D. Adams, G. Parrott, C. Bauer, T. Sant, L. Manning, M. Jones, B. Rogers, D. McCorkle, and T. L. Ferrell, “Nanowatt chemical vapor detection with a self-sensing, piezoelectric microcantilever array,” Applied Physics Letters, vol. 83, pp. 3428-3430, October 2003], piezoresistive [N. Abedinov, P. Grabiec, T. Gotszalk, T. Ivanov, J. Voigt, and I. W. Rangelow, “Micromachined piezoresistive cantilever array with integrated resistive microheater for calorimetry and mass detection,” Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films, vol. 19, pp. 2884-2888, November-December 2001] or capacitive [D. R. Baselt, B. Fruhberger, E. Klaassen, S. Cemalovic, C. L. Britton, S. V. Patel, T. E. Mlsna, D. McCorkle, and B. Warmack, “Design and performance of a microcantilever-based hydrogen sensor,” Sensors and Actuators B-Chemical, vol. 88, pp. 120-131, January 2003] schemes are employed.
For example, microcantilevers with a compound immobilized on the surface on the free end have been used to detect and screen receptor/ligand interactions, antibody/antigen interactions and nucleic acid interactions (U.S. Pat. No. 5,992,226, issued on Nov. 30, 1999). Deflection was measured using optical and piezoelectric methods.
Microcantilevers can also be used to measure concentrations using electrical methods to detect phase difference signals that can be matched with natural resonant frequencies (U.S. Pat. No. 6,041,642, issued Mar. 28, 2000.) Determining a concentration of a target species using a change in resonant properties of a microcantilever on which a known molecule is disposed, for example, a macromolecular biomolecule such as DNA, RNA, and protein, is described in U.S. Pat. No. 5,763,768
Another method and apparatus for detecting and measuring physical and chemical parameters in a sample media uses micromechanical potentiometric sensors (U.S. Pat. No. 6,016,686, issued Jan. 25, 2000). Detection of a chemical analyte is described in U.S. Pat. No. 5,923,421, issued Jul. 13, 1999. Magnetic and electrical monitoring of radioimmune assays, using antibodies specific for target species which cause microcantilever deflection, e.g., magnetic beads binding the target to the microcantilever, are described in U.S. Pat. No. 5,807,758, issued Sep. 15, 1998.
U.S. Pat. Nos. 6,096,559 issued Aug. 1, 2000, and 6,050,722 issued Apr. 18, 2000, describe fabrication of a microcantilever, including use of material such as ceramics, plastic polymers, quartz, silicon nitride, silicon, silicon oxide, aluminum oxide, tantalum pentoxide, germanium, germanium dioxide, gallium arsenide, zinc oxide, and silicon compounds. Coating of micromechanical sensors with various interactive molecules is described in U.S. Pat. No. 6,118,124, issued Sep. 12, 2000.
However, these and other known methods suffer from lack of analyte specificity, offer small resistance changes and/or draw continuous power. In addition, an intermediate transduction step, often optical or mechanical, is used. Further, these measurements are susceptible to ambient interference and require additional power-consuming electronic circuitry. It has been suggested that the requirement of relatively thick sensing layers, typically on the order of several micrometers, in soft-matter-based sensors (i.e. polymers) is another major disadvantage as it limits their incorporation into nano-scale devices [S. Singamaneni, M. C. LeMieux, H. P. Lang, C. Gerber, Y. Lam, S. Zauscher, P. G. Datskos, N. V. Lavrik, H. Jiang, R. R. Naik, T. J. Bunning, and V. V. Tsukruk, “Bimaterial microcantilevers as a hybrid sensing platform,” Advanced Materials, vol. 20, pp. 653-680, February 2008]. The best cases of “ultrathin” sensors have thicknesses greater than 300 nm. Furthermore, such systems show only partial selectivity, which is based on the rate of different molecules diffusing into polymer layers.
Therefore, there is a need for improved chemical sensors which, for example, do not require optical, piezoelectric, piezoresistive or capacitive measuring schemes; are not susceptible to ambient interference; and do not require additional power-consuming electronic circuitry.